Seaman in base? It's feasible (6)
I believe the answer is:
viable
'feasible' is the definition.
('viable' can be a synonym of 'feasible')
'seaman in base?' is the wordplay.
'seaman' becomes 'ab'.
'in' indicates putting letters inside.
'base?' becomes 'vile' ('vile' can be a synonym of 'base').
'ab' placed inside 'vile' is 'VIABLE'.
'it's' acts as a link.
(Other definitions for viable that I've seen before include "having potential to succeed" , "Able to function, grow normally" , "Sound" , "Possible and practical" , "(Of plan) capable of working" .)